1. Field of the Invention
The present invention relates to an electrolytic capacitor to be used for various kinds of electronic appliances. Particularly, the present invention relates to an improvement of lead wires led out from a capacitor element.
2. Description of the Related Art
Conventionally, as capacitors for high frequency ranges, laminated ceramic capacitors have been used. However, these capacitors have a relatively small capacitance. Therefore, as compact capacitors having a large capacitance and a relatively low equivalent series resistance (hereinafter, abbreviated as ESR), electrolytic capacitors employing a valve metal such as tantalum and aluminum have been widely used. In particular, solid electrolytic capacitors employing electrically conductive polymers such as polypyrroles, polythiophenes, and polyanilines, or TCNQ (7,7,8,8-tetracyanoquinodimethane) complex salts as their electrolytes have attracted attention.
A solid electrolytic capacitor of the foregoing type is fabricated in the following manner. First, an anode foil formed by etching and anodizing an aluminum foil and a cathode foil formed by etching an aluminum foil are respectively prepared. Then, an anode lead tab welded with a lead wire and a cathode lead tab welded with a lead wire are fixed to the anode foil and the cathode foil, respectively. Thereafter, the anode foil and the cathode foil are wound around in a cylindrical form with a separator interposed therebetween to form a capacitor element. Further, cut edge formation and a heat treatment are carried out for the capacitor element. Next, the capacitor element is immersed in a solution containing a monomer and an oxidizing agent, and thereafter, thermal polymerization is effected, whereby a conductive polymer layer (a solid electrolyte layer) is formed between the electrode foils of the capacitor element. Subsequently, the capacitor element is accommodated and secured in an outer casing having an opening part at one end side thereof, then the opening part of the outer casing is sealed with a sealing body having through-holes to lead out the respective lead wires, and further an aging process is performed. After a seat plate is placed, the respective lead wires are led out to the outside through lead inserting through-holes of the seat plate placed on the sealing body side and press-worked and bent to form electrode terminals, so that a solid electrolytic capacitor is completed.
In an electrolytic capacitor described above, respective lead wires of the electrolytic capacitor are electrically connected with electrode foils by welding one end sides of the lead wires with aluminum boss members of lead tab terminals connected with the electrode foils. Further, the other end sides of the lead wires led out through lead inserting through-holes of a seat plate are soldered with a mount board as electrode terminals. Accordingly, in the electrolytic capacitor, one end sides of lead wires are required to be excellent in weldability with the boss members made of aluminum and the other end sides of the lead wires are required to be excellent in solder wettability. Therefore, in general, lead wires having plating layers such as a silver plating layer, a tin plating layer, or a tin-bismuth based alloy plating layer on the surface of a substrate such as a copper wire are used.
However, in the case of forming a solid electrolyte layer containing an electrically conductive polymer such as a polythiophene type conductive polymer between electrode foils, since monomers are thermally polymerized in a capacitor element to which lead wires are connected as described above, a sulfidizing gas is generated from the monomers and dopant agents at the time of the thermal polymerization. In a case where such a sulfidizing gas is generated, in a lead wire having a silver plating layer, the silver plating layer is polluted with the sulfidizing gas, resulting in a problem that the silver plating layer is discolored and solder wettability of the lead wire is lowered. It is supposed to be possible to wash the lead wire after the thermal polymerization; however the silver plating layer is also discolored with a sulfur component in air and therefore in industrial production, there is also a case where an electrolytic capacitor is left for a long time until the electrolytic capacitor is soldered with a mount board after production of the electrolytic capacitor. From this reason, there is a problem that the silver plating layer discolors with the lapse of time and it results in decrease of solder wettability.
On the other hand, with respect to a capacitor element to which lead wires having a tin plating layer or a tin-bismuth based alloy plating layer are connected, the plating layer tends to be melted due to high temperature at the time of the thermal polymerization in production process or heat treatment of electrode foils and a separator. Therefore, there is a problem that a diameter of the lead wire tends to become uneven. Further, there is also a problem that in lead wires having a tin plating layer or a tin-bismuth based alloy plating layer, whisker occurs easily. Furthermore, the whisker tends to be generated easily by using an electrolytic capacitor under high temperature environments. Therefore, in the case of using lead wires having a tin plating layer or a tin-bismuth based alloy plating layer, short circuit may possibly be caused between an anode lead wire and a cathode lead wire.
As described above, defective products tend to be produced by using lead wires coated with any of the plating layers and it may possibly result in a problem of decrease of productivity and a problem of defective short circuit.
In a chip-shaped electrolytic capacitor, in order to improve solderability and ESR, formation of a copper plating layer, a nickel plating layer, a palladium plating layer, and a gold plating layer on the surface of a lead frame made of a nickel-iron based alloy in a thickness of about 100 μm has been also proposed. (See, for example, Japanese Unexamined Patent Publication No. 11-283870) However, there is still a problem that even the plating layers formed on this lead frame fail to attain sufficient weldability between boss members made of aluminum and lead wires having small diameters.